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Quiz: Fiber, Fatty Acids, and the Blood-Brain Barrier

Test your understanding of Chapter 6. Choose the best answer, then click Show Answer to check.

Question 1

Which short-chain fatty acid is usually the most abundant, in both the colon and the blood?

  1. Acetate
  2. Butyrate
  3. Propionate
  4. Lactate
Show Answer

The correct answer is A. In the 1987 human study by Cummings and colleagues, acetate was the main SCFA in every sample, from colon contents to peripheral blood. Butyrate (B) gets a lot of attention, but most of it is used up by the colon lining, so much less of it reaches the blood.

Concept Tested: Acetate Bloom's Level: Remember

Question 2

Where in the human digestive tract does most fiber fermentation take place?

  1. In the stomach, where acid breaks fiber apart
  2. Throughout the small intestine, evenly from start to end
  3. In the first part of the colon, the cecum and ascending colon
  4. Only in the rectum, just before stool leaves the body
Show Answer

The correct answer is C. SCFA levels were low at the end of the small intestine and highest in the cecum, so most fermentation happens at the start of the colon. Option B is a common mix-up: the small intestine absorbs most digestible food, but fiber passes through it largely intact.

Concept Tested: Fermentation Bloom's Level: Remember

Question 3

What is a colonocyte?

  1. A type of bacterium that ferments fiber in the colon
  2. An epithelial cell that lines the inside of the colon
  3. An immune cell in the brain that responds to SCFAs
  4. A hormone released when fiber reaches the colon
Show Answer

The correct answer is B. Colonocytes are the cells lining the colon, and in mice they use bacterial butyrate as their primary fuel. Option A confuses the cell that uses butyrate with the microbes that make it.

Concept Tested: Colonocyte Bloom's Level: Remember

Question 4

In the 1987 autopsy study, total SCFAs fell from about 375 micromoles per liter in the portal vein to about 79 in peripheral blood. What best explains this drop?

  1. SCFAs break down on their own within seconds of leaving the colon.
  2. The kidneys filter SCFAs out before blood reaches the liver.
  3. The blood-brain barrier absorbs most SCFAs from the blood.
  4. The colon lining uses much of the butyrate and the liver takes up much of the propionate, so less is left for the rest of the body.
Show Answer

The correct answer is D. The changing mix of SCFAs from colon to portal blood to hepatic vein showed greater uptake of butyrate by the colon lining and propionate by the liver. Option C is wrong because the brain receives only a small share of blood SCFAs; the big drops happen in the gut wall and liver.

Concept Tested: Short-Chain Fatty Acid Bloom's Level: Understand

Question 5

How does butyrate change which genes a cell switches on?

  1. It cuts DNA at specific genes, like a pair of scissors.
  2. It blocks histone deacetylases, leaving DNA more loosely wrapped and some genes easier to switch on.
  3. It replaces the cell's DNA with bacterial genes.
  4. It binds to the outside of the cell and stays there, never entering.
Show Answer

The correct answer is B. By blocking HDACs, butyrate leaves more acetyl tags on histones, loosening the DNA so some genes are easier to read. In mice, this happened at the Foxp3 gene that makes T cells become regulatory T cells. Option D describes receptor signaling, which is a separate way SCFAs act.

Concept Tested: Gene Regulation by Butyrate Bloom's Level: Understand

Question 6

Why do scientists say a "high" result on a common commercial zonulin blood test does not show that someone has a leaky gut?

  1. Independent studies found that widely used kits detect other blood proteins, not zonulin itself.
  2. Zonulin is only found in mice, so it can't be measured in people.
  3. Zonulin levels are always high in healthy people.
  4. The test measures stomach acid instead of blood proteins.
Show Answer

The correct answer is A. Scheffler and colleagues (2018) and Ajamian and colleagues (2019) found that commercial kits picked up proteins like properdin, haptoglobin, and complement C3 instead of zonulin. Option B is wrong: zonulin is studied in people, but the popular tests don't measure it reliably.

Concept Tested: Zonulin Bloom's Level: Understand

Question 7

Sam runs a hard 10-mile training run, and the next day a research team measures his small intestine permeability with a sugar test. Based on this chapter, what result is most likely, and what does it mean?

  1. Permeability will be zero, because exercise seals tight junctions completely.
  2. Permeability will be higher, which proves he has leaky gut syndrome and needs a supplement.
  3. The sugar test can't be used on athletes.
  4. Permeability may be higher than usual, but this is a normal response to endurance exercise, not a disease.
Show Answer

The correct answer is D. Camilleri's review lists endurance exercise as one of several stressors that increase intestinal permeability in people. Option B makes the leap the chapter warns against: a raised measurement isn't a diagnosis, and "leaky gut syndrome" isn't a recognized condition.

Concept Tested: Intestinal Permeability Bloom's Level: Apply

Question 8

A student uses the Fiber to Butyrate MicroSim. At 30 grams of fiber a day with very low microbe diversity, the colonocyte fuel meter is lower than at 20 grams with high diversity. Which real-world idea is the model illustrating?

  1. Fiber has no effect on SCFA production.
  2. Only the amount of fiber matters; microbes are all the same.
  3. How much butyrate is made depends on both the fiber eaten and which microbes are present to ferment it.
  4. High microbe diversity causes the blood-brain barrier to tighten in people.
Show Answer

The correct answer is C. Fermentation needs both the fiber and the microbes that can ferment it, which is why the same meal can produce different amounts of SCFAs in different people. Option D is wrong because the blood-brain barrier effect has been shown only in germ-free mice, and the sim labels it that way.

Concept Tested: Butyrate Bloom's Level: Apply

Question 9

A headline says, "Gut bacteria keep your brain's barrier sealed." It is based on Braniste and colleagues (2014). Which statement best analyzes the gap between the study and the headline?

  1. The study compared germ-free mice with normal mice, so it can't show what happens in people, who always have microbes.
  2. The study was done in people, so the headline is accurate.
  3. The study found no difference between germ-free and normal mice.
  4. The study proved that a specific probiotic seals the human blood-brain barrier.
Show Answer

The correct answer is A. The study found a leakier BBB in germ-free mice and tighter barriers after adding microbes, but no human data were collected, and the comparison was between no microbes and normal microbes. Option D overstates it: no probiotic product was tested in people.

Concept Tested: BBB Permeability Bloom's Level: Analyze

Question 10

Cani and colleagues (2007) found that a high-fat diet raised blood LPS two to three times in mice, which they called metabolic endotoxemia. A wellness site says, "This proves fatty food leaks bacterial toxins into your blood and makes you sick." What is the strongest criticism of this claim?

  1. LPS is not made by bacteria, so the study was mislabeled.
  2. The study was in mice, and human studies on blood LPS are mostly observational, so it doesn't prove the same effect in people.
  3. The study found that LPS lowers inflammation.
  4. Metabolic endotoxemia means a large, deadly LPS level, which the mice did not have.
Show Answer

The correct answer is B. The 2007 findings are strong mouse evidence, but human links between diet, blood LPS, and health are mostly correlations that can't show which came first. Option D gets the definition backward: metabolic endotoxemia means a small, chronic rise in LPS, not a severe infection-level spike.

Concept Tested: Metabolic Endotoxemia Bloom's Level: Analyze